What do you base your guesstimate on? Freeze plugs themselves are very inexpensive, and changing one out is typically as easy as tap out the bad one, and tap in the new one. Especially since you can see it, it may not be too difficult to get to. Getting the right size is the most important thing, since it is a friction fit.
But, you will need to replace it. It is a slow leak now, but they tend to grow, and may blow out at any time, leaving you stranded. The leak is a warning you caught early.